The imaginal discs of Drosophila provide a useful model system for analyzing the genetic control of cell determination and phenotypic expression. These problems will be approached by employing in vivo culture of imaginal disc cells, by producing genetic mosaics to identify environmental and clonal aspects of cell commitments, and by using conditional (i.e., temperature sensitive) mutants in which the ultimate phenotypic effect of the gene can be manipulated by the experimenter. Special emphasis will be placed on the initiation of determination during embryogenesis, the genetic control of determination by homoeotic control genes, and sex determination in cells of triploid intersexes. Temperature sensitive maternally influenced lethals will be sought in order to investigate the role of the maternal genome in laying down the cortical cytoplasm, which probably effects initial embryonic commitments. Problems investigated will include the developmental period in which specific biases are initiated, the number of cells involved in determinative events, the nature of inheritance for determined states, whether the effect of a control gene is retained after the gene is removed from a cell, the "infective" nature of specific determined states, and the role of cell division in determination. The results promise to illuminate the genetic control of congenital abnormalities and aberrant growth in higher organisms.